I think that’s probably something that you would ultimately have to figure out for yourself. I’m also currently writing a horror game, so I don’t want to just flat out give out all of my ideas (which is why I say you may have to figure it out on your own). But what I can say is this. Horror games lately to me aren’t that fun to play (at least, AAA). They either have too many jump scares where the other 90% of the game isn’t scary at all, or they just aren’t that fun to play.
*In creating your game, think of what type of game you want your audience to play. Is it a survival game? Is it a checkpoint-to-checkpoint game (such as Left 4 Dead)? Base defense / scavenge for supplies kind of game? Left 4 Dead had no jumpscares really from what I remember, but that game is extremely scary to play solo and also with other people when you start falling behind / getting left behind. The feeling of being “in the back” defenseless is scary especially when you start hearing growls.
*As far as the “Most Important,” that’s up to you to decide but one thing is that giving the player too much power, too much ammo, or making mechanics such as 1-hit sneak attacks too readily-available will break immersion, because that right there will allow the player to feel too powerful; and if they feel too powerful, they won’t be scared. The player needs to fear for their live in one way or another, at many points throughout the game.
You’d have to decide if you want your game to focus on a story, focus on figuring out why something is haunted, or whatever you want your game to be about and then think about what you’d want you yourself, and your players to experience. Also, something I noticed when playing Evil Within (a good game in my opinion) is that spawns of the enemies were never static. Sometimes they would be there when I reloaded, and sometimes they wouldn’t be and will have spawned somewhere else. That was extremely frightening for me because the thought of “Ok, I know what’s behind this door” causes me to feel safe because I can prepare for it; but the moment they’re not actually behind that door and are somewhere else, I no longer am able to prepare myself to be safe and it creates fear.